1. Field of the Invention
The present invention relates to the fibre optic transmission of analog signals and more particularly to the combination of a laser with an optical modulator to transmit analog signals.
2. Description of the Background
This combination is generally preferred to the use of direct-modulation lasers. Indeed, regarding direct-modulation lasers, the variation in current generating the amplitude modulation influences the wavelength emitted by the laser, and this variation, even when slight, degrades transmission performance if it results in the chromatic dispersion of the optical fibre.
The combination of certain lasers with an external optical modulator may however have another drawback due to the low-frequency, relaxation noise. This relaxation noise is manifested as a fluctuation in power emitted on the order of a few percent and corresponds to a narrow low-band spectrum. Such is the case for Glass-Erbium type lasers for which the central frequency of this spectrum is around 200 kHz. This noise, represented in FIG. 1a, causes a disturbance as described below in greater detail.
The modulating input of the optical modulator receives a signal consisting of one or more carriers (e.g., amplitude-modulated carriers). An example of this signal, represented in FIG. 1b, consists of an audio and video carrier. The frequency spectrum of the video carrier corresponds to a vestigial sideband amplitude modulation (referred to as VSB-AM). These 2 modulated carriers represent a channel within the field of television signal transmissions, and the modulating signal transmitted to the optical modulator can consist of a set of adjacent channels so as to constitute a multichannel frequency multiplex of vestigial sideband amplitude-modulated television signals. Multiplication of the optical signal with this RF modulating signal transposes the noise of the laser over to around the carriers of the RF signal, as represented in FIG. 1c. This noise ends up in the frequency sidebands if the latter have a width greater than a few tens of kilohertz, this being the case for the video modulating signal. After demodulation, the baseband video signal encounters the presence of noise of double power since the two noise "lines" around the video carrier are correlated while the spectrum of the video signal has reduced sideband. This noise level then exceeds the threshold of visibility of the video signal on the screen by a value of up to 10 or 12 dB, thus creating a "scribbling" effect on the image.
A "Feed Forward" solution is known that involves inserting, between the laser and the external modulator, a device for regulating the emitted power. This device is represented in FIG. 2. A coupler 2 taps off a portion of the power at the output of the laser 1 and to transmits it to an optical receiver 3 (with a built-in amplifier) which converts this optical signal into an electrical signal. The electrical signals controls a first external optical modulator 4 which acts as regulator.
A suitable choice of the coupling and gain values of the optical receiver enables the noise of the laser to be reduced effectively before the beam is transmitted to the second external optical modulator 5 receiving the RF modulating signals.
A first drawback of this solution relates to the insertion losses of the coupler and of the optical regulating modulator which greatly reduce the usable optical power of the laser. This occurs because the optical modulator, by nature, halves the power. Moreover, even without taking those insertion losses into account, the output power available after regulation is at most equal to the minimum value of the fluctuating power delivered by the laser. Another drawback relates to the setup of the device. The gain of the regulating chain symbolized by the optical receiver 3 has to be accurately adjusted. Additionally, the correction signal has to be perfectly in phase with the noise to be corrected. Finally, the cost of the optical modulator necessary for the power regulation is a consideration cost.